How often should I change the oil in my 4-stroke outboard?

4-stroke outboard oil change interval · Symptom: You're not sure when your 4-stroke outboard is due for an oil change, the oil looks dark or smells like gas, or the level on the dipstick seems to be rising.

Quick answer: Most 4-stroke outboards need an engine oil change every 100 hours or once a year, whichever comes first, plus a first change after break-in (Yamaha says after the first 10 hours; check your manual). Some Yamaha manuals say every 100 hours or 6 months. Change it more often if you troll a lot, idle for long periods, or make lots of short trips, because fuel and moisture build up in the oil. Use NMMA FC-W certified 4-stroke marine oil, change the filter if your engine has one, and check the level with the motor upright and level.

Likely causes of oil wearing out early (most to least common)

Long idling, trolling, or short trips: the engine never gets hot enough to burn off fuel and moisture, so the oil gets diluted (Mercury calls this oil dilution)
Skipping the annual change because the engine has few hours on it
Wrong oil: car oil or oil without the NMMA FC-W rating
Missed break-in oil change on a new or rebuilt engine
Water in the oil (milky oil) from overheating, a head gasket, or submersion

Safe owner checks & fixes

Check the level before every trip with the outboard upright (not tilted) and level. Yamaha says to stop the engine and wait a few minutes before reading the dipstick or sight window.
Look up your interval in the owner's manual. Typical: Mercury FourStroke every 100 hours or once a year; Yamaha every 100 hours or every 6 months (manual) / annually or 100 hours (Yamaha's maintenance chart), with a first change after the break-in period.
Change the oil while it's warm (not hot). Drain it, put a new gasket on the drain screw, and torque it to spec. Replace the oil filter if the engine has one.
Refill with the amount and grade in your manual (for example, Mercury recommends NMMA FC-W 10W-30 for general use). Don't overfill: if the level is over the upper mark, drain down to it.
Start the engine, make sure the low oil pressure warning goes off, and check for leaks. Then stop, wait about 3 minutes, and recheck the level.
If the level keeps rising or the oil smells strongly of fuel, change it now and shorten your interval. Milky or tan oil means water: don't run it (see the dunked-motor and overheating pages) and get it checked.

Parts / tools

4-stroke marine oil (NMMA FC-W) in the grade your manual lists, oil filter and filter wrench (if equipped), new drain screw gasket, drain pan, rags, torque wrench, owner's manual.

Call a marine mechanic when

The oil is milky or has metal flakes in it; the low oil pressure warning stays on; oil keeps getting diluted with fuel even with frequent changes; or you can't reach the drain or filter safely.

Sources

Values not stated here are model-specific; check your owner's manual.